Crack growth model for pipelines exposed to concentrated carbonate–bicarbonate solution with high pH

The crack tip strain rate (CTSR) and the dissolution/repassivation kinetics are the parameters controlling high pH SCC of pipeline steels because the repeated film rupture is the dominative mechanism. The CTSR is mainly produced by the crack tip advance and cyclic load. Theoretical expressions of the CTSR are given to account on these factors. After the anodic current density determined from a polarization curve measured with a potential scanning velocity 1 V/min and the repassivation kinetic exponent obtained by the strained electrode method are utilized, the crack growth model proposed gives a reasonably good prediction to the crack velocity experimentally observed. A numeric simulation in a 0.5 M carbonate + 0.5 M bicarbonate solution indicates that the effect of mass transfer within crack on the crack growth is negligible when the crack velocity is below 10−9 m/s. Various factors affecting the SCC are discussed.

Research highlights
► Corrected the mistake in applying Shoji’s equation in crack tip strain rate estimation and proposed a new one to estimate the crack tip strain rate produced by crack tip advance.
► Proposed the equation for estimation of crack tip strain rate produced by the cyclic loads due to the internal pipe pressure fluctuations.
► Built a physical model for crack growth of pipeline steel in concentrated carbonate–bicarbonate solution with high pH in accordance with the crack tip strain rate equation, anodic dissolution/repassivation kinetic parameters determined by experiments. The comparison with the existing experimental data indicated that the new model can provide a reasonably good prediction on the various factors on the high pH SCC of pipelines.